1. Field of the Invention
The present invention relates to a technology of signal receiving and, more particularly to a method and apparatus for receiving a spread spectrum signal.
2. Description of the Related Art
Wireless local area network (LAN) that complies with the IEEE 802.11b standard is practiced as a spread spectrum communications system using a radio frequency of 2.4 GHz band. The IEEE 802.11b wireless LAN enables a maximum transmission rate of 11 Mbps using complementary code keying (CCK). The Radio Law prescribes the bandwidth of wireless LAN to be 26 MHz. Therefore, the maximum chip rate in a direct sequence scheme is also 26 Mcps. Assuming that the chip rate of 26 Mcps is band-limited by an ideal Nyquist filter, the sampling frequency required of a D/A converter is 40 MHz. Also, strict band limitation after the D/A conversion is necessary. Therefore, band limitation using a Nyquist filter is not practical. Instead of using a Nyquist filter for band limitation, analog filtering subsequent to the D/A conversion is used for band limitation, resulting in a maximum chip rate of 11 Mbps. In a receiver adapted for CCK modulation, a plurality of waveform patterns for a transmitted signal are generally prepared. A signal having a waveform that best matches the waveform of the received signal is defined as a demodulation result (See Reference (1) in the following Related Art List, for instance).
Related Art List
(1) Japanese Patent Application Laid-Open No. 2003-168999.
In demodulating a CCK modulated signal, i.e. in demodulating a Walsh-coded signal, a plurality of correlations are determined from a received signal, using fast Walsh transformation (FWT) computation. The largest correlation is identified from a plurality of correlations. A combination of transmitted signals providing the largest correlation is reconstructed. If an error is included, however, in correlations determined as a result of FWT computation due, for example, to noise or multipath transmission, a combination of signals not actually transmitted may be selected. Since a wireless LAN apparatus is desirably small, internal processes should be simplified as much as possible.